1. Field of the Invention
This invention relates to a fluid pressure control system for controlling a plurality of actuators. More specifically, it relates to a fluid pressure control system suitable for, but not exclusively, controlling the actuation of a fluid pressure cylinder mechanism for vertically moving a boom and a fluid pressure motor for swinging an upper swing frame in a hydraulic excavator.
2. Description of the Prior Art
A hydraulic excavator is provided with an upper swing frame mounted pivotally on a moving undercarriage, a boom mounted on the upper swing frame for free vertical movement and a bucket mounted pivotally on the end portion of the boom via an arm. The upper swing frame is caused to swing by the action of the fluid pressure motor, and the boom is actuated vertically by the extension and retraction of a fluid pressure cylinder mechanism for the boom. The bucket is actuated by the extension and retraction of a fluid pressure cylinder mechanism for the bucket. The following problems to be solved, which will be described in detail later on, exist with the fluid control system provided in a conventional hydraulic excavator. First, when the rotation of the fluid pressure motor is restricted by the resistance of an external load, the fluid pressure supplied to the fluid pressure motor abruptly increases to swing the upper swing frame with a strong torque. Consequently, the fine-controllability of the upper swing frame is aggravated. Secondly, when, for example, the fluid pressure cylinder mechanism for the boom and the fluid pressure motor for swing are actuated in full motion simultaneously, the rotation of the shaft of the fluid pressure motor tends to be regulated at the time of starting the swinging of the upper swing frame. This regulated rotation abruptly increases the fluid pressure supplied to the fluid pressure motor, and the upper swing frame moves at a relatively high speed. Thus, the swinging speed of the upper swing frame becomes much faster than the lifting speed of the boom.